Present digital data communication systems for transferring digital data generally are classified either as synchronous or asynchronous systems. Asynchronous transmission involves sending equal length bursts of data, with a common data character format for this purpose comprising a start bit, the data bits (with or without a parity bit) and one or more stop bits. No clock information is sent with the data since the receiver easily resynchronizes at the beginning of each character. A large percentage of overhead bits are required for asynchronous transmission.
Synchronous transmission involves sending blocks of data bits together with synchronizing, or framing, characters at, say, one hundred byte intervals of data. With synchronous transmission, much lower percentage of overhead bits are required. Now, however, the data must include therein the synchronizing, or framing, characters. Also, much more complex error checking schemes are required for synchronous systems compared to asynchronous systems if the loss of large blocks of information is to be avoided when transmission errors occur and synchronization is lost. With the present invention, the only overhead bits required are word parity bits which often are transmitted in any case.
Reference here is made to copending U.S. Pat. Application Ser. No. 311,782 filed concurrently herewith and assigned to the same assignee as the present invention.